Summary Northwest Territorial 962 (NWT962), a Boeing 737-210, was en route from Rankin Inlet, Northwest Territories, on an instrument flight rules (IFR) flight at flight level (FL) 310 via Churchill, Manitoba, direct to Winnipeg. Northwest 69 (NW69), a Boeing 747-451, was en route from Detroit, USA, also on an IFR flight at FL310, via Red Lake, Ontario, and airway NCA20 to Kansai, Japan. (See Appendix A.) As NWT962 approached the southern boundary of the Winnipeg North High (North) sector, the radar controller observed an unidentified target converging with NWT962's track. The controller identified the target as NW69, and at 1455:19 central daylight saving time (CDT), re-cleared NWT962 to FL290. NWT962 commenced descent from FL310 at 1455:58 CDT with 9.5 nautical miles (nm) horizontal separation from NW69, and levelled at FL290 at 1456:32 CDT with about 3 nm horizontal separation. The minimum required separation in the Class A airspace where the incident occurred is 5 nm horizontal or 2,000 feet vertical separation. Ce rapport est galement disponible en franais. Other Factual Information Air traffic control (ATC) in the area of the occurrence is provided by the Winnipeg Area Control Centre (ACC). The two flight paths crossed about 160 nm northeast of Winnipeg at 1456:48 CDT, about 6 nm north of the boundary between the North sector and the Winnipeg East (East) sector. At that point, the two aircraft had about 500 feet horizontal and 2,000 feet vertical separation. NW69 crossed from the East sector into the North sector, but the East controller had not yet handed off the flight to the North radar controller. The East sector controllers have two methods of determining an aircraft's position: the flight progress strips, and the radar display. There was no posted time estimate for a co-located position for the two flights. To recognize the conflict between NWT962 and NW69, it would have been necessary to use NWT962's Gimli estimate to calculate the time of its crossing of NCA 20, NW69's route of flight. Neither the East data controller nor the radar controller recognized the conflict between NWT962 and NW69 from an examination of the flight progress strips. Both were aware of the procedure for calculating the separation between the flights. The East sector was operated by a radar controller and a data controller until 1449 CDT, when the data controller left for a rest break and the radar controller assumed both positions. Traffic in the sector was judged to be light at that time. The East controller was taking a position estimate by telephone when the North controller indicated that he was descending NWT962 and that NW69 should be left at FL310. The East controller had planned to hand off NW69 to the North controller at or immediately before the aircraft reached the sector boundary. However, the East controller usually did not hand off most aircraft on NCA20 to the North sector before the sector boundary because of potential communications difficulties, as the next remote communications outlet (RCO) near that route is located at The Pas. Flights attempting to use the RCO at The Pas from a position in the East sector often find The Pas RCO to be out of range. Such flights could be assigned to the other available RCO frequency at Island Lake, but they would be out of range of Island Lake several minutes later, and the North controller would then be required to switch the flights to the RCO at The Pas. The East controller considered it desirable to reduce unnecessary workload on aircraft crews and controllers in other sectors by delaying the hand-off of aircraft entering the North sector until the aircraft were within communications range of The Pas. The North radar controller's instruction to NWT962 at 1455:22 CDT was: Territorial nine six two maintain flight level two nine zero. Start descent now. NWT962 replied: Territorial 962 we're leaving three one zero. At 1455:52 CDT, NWT962 asked: Centre it's nine sixty two. Do you want us to start our descent now? The North radar controller replied: Territorial nine six two affirmative. Two nine zero right now please. At the time they received the first message from the controller, the NWT962 crew was not expecting a new clearance from the controller, and discussed the situation in the ensuing 30 seconds. During that time, the two aircraft closed from a horizontal separation of 19 nm to 12 nm. Neither crew member recalled hearing the clearance limit of FL290 or the urgency of the instruction to descend. Pilots and controllers form mental pictures of the relative positions of aircraft and reporting points to assist them in understanding the overall traffic situation. Pilots and controllers who are involved in a communication exchange usually process the received information using mental expectations that seem most appropriate for the activity that they are performing at the time of the communication. If the message is unexpected or unusual, then the mental expectation held by the information receiver may hinder the understanding of the message, and could delay the response to the new information. At the time that the radio transmissions were made, the crew of NWT962 was in cruise flight conditions approximately 160 nm from their destination. They were not aware of any impending traffic conflict, had not requested a descent, and were not anticipating a descent clearance from that point. The Air Traffic Control Manual of Operations (ATC MANOPS), section 507.1 instructs controllers to, Issue a safety alert to traffic if you are aware that the aircraft is at an altitude which...places it in unsafe proximity to...another aircraft. The phraseology to be used in such a situation is, Traffic alert (position of traffic if time permits)...climb/descend (specific altitude if appropriate) immediately. The procedure set out in this section is reportedly used mostly in connection with mixed IFR/visual flight rules (VFR) traffic. The North radar controller set his indicator module (IM) to show only the data blocks of the aircraft under his control. On this setting, his IM showed only an octagonal present position symbol, but no data tag, for aircraft controlled by the North Low sector or by an adjacent sector such as East. At the time of the occurrence, the North IM was set on 256 nm scale, the maximum available setting. The North sector is geographically larger than most other sectors controlled by the Winnipeg ACC, and even at the 256 nm setting, the sector depiction occupies most of the space available on an IM. However, if the North sector is centred on the IM display, part of the East sector airspace will be displayed in the lower left corner of the IM. NWT962's track was about 190 degrees magnetic () at the time of the occurrence. The flight crew had been in radar contact with Winnipeg ACC controllers since entering the North sector near Churchill. NW69's track was about 310, and the flight had been in radar contact with Winnipeg ACC controllers in the Marathon and Dryden sectors before entering the East sector about 40 nm northwest of Red Lake. Above FL290, aircraft on tracks from 180 to 359 are assigned cruising altitudes at 4,000 foot intervals, beginning at FL310. At the time of the occurrence, the Winnipeg North specialty, comprising the data and radar controller positions in the Winnipeg High, Winnipeg Low, and Trout Lake sectors, was staffed with three controllers and two supervisors. No controllers from the other specialties were qualified in the Winnipeg North specialty. One of the two supervisors was working the Trout Lake sector, and the other was on a break. At 1422 CDT, the two controllers working the North Low sector observed that the single controller working the North sector was experiencing a period of heavy workload. One of the North Low controllers then moved to the North radar position to assist that controller, who then assumed the North data position. The incoming controller received no status briefing on assuming the North radar position; the incumbent North controller was reportedly too busy to provide one. A status briefing is to apprise an incoming controller of the existing and expected traffic in a sector and of any anticipated conflicts. On the morning of the occurrence, staffing in the North sector was judged by the ACC shift manager to be sufficient for the day's traffic requirements. The controllers in the Winnipeg North specialty have three methods of determining an aircraft's position: the flight progress strips, the radar display, and the Northern Airspace Display System (NADS). The NADS is a computer-driven, representational display system which plots position estimates and crossing points for aircraft beyond the range of radar coverage. NADS helps controllers to recognize and avoid traffic conflicts at crossing points. The system depends on timely and accurate input of aircraft positions. The first estimate for NWT962's position abeam Gimli was 1519 CDT, based on that flight's departure time and estimated time en route. NADS issued a track crossing warning for NWT962 and NW69 based on this original estimate, and indicated that the two aircraft would cross with six minutes separation. ATC MANOPS directs controllers to post red W warning indications on flight progress strips to identify conflicts with other aircraft. Neither North controller marked a warning indication on either aircraft's flight progress strip. Both North controllers believed that a crossing with six minutes separation could be monitored on radar, without the need to change the clearances of the aircraft. After NWT962 came into radar contact and its position and speed could be determined more accurately, the North radar controller calculated that the flight would be abeam Gimli at 1513 CDT. He marked the revised estimate on the North sector's flight progress strip and passed it to the East data controller, who marked it on the East sector's NWT962 flight progress strip. The North data controller then continued updating NADS but did not enter the revised time estimate into NADS before the occurrence. There was no posted time estimate for a co-located position for the two flights. As in the East sector, to recognize the conflict between NWT962 and NW69, the North sector controllers would be required to use NWT962's Gimli estimate to calculate the time of its crossing of airway NCA 20, NW69's route of flight. Neither North controller could recall performing this calculation. Both aircraft were equipped with traffic and collision avoidance systems (TCAS), although TCAS systems are not required by Canadian regulation. The TCAS system in NWT962 reportedly activated when that aircraft was descending through 29,200 feet, at a range of 5 nm from NW69, with a time to closest point of approach (CPA) of about 35 seconds. The system issued an aural traffic alert, along with position and altitude information on NW69. The TCAS system in NW69 reportedly issued an aural traffic alert about 40 seconds before the CPA between the two aircraft, and when the crew selected the TCAS screen on their navigation display, it indicated the position and altitude of NWT962. In both cases the TCAS indications preceded and corresponded with the visual sighting of the aircraft traffic by the crews. The radar modernization project (RAMP) radar system in use at the Winnipeg ACC is not capable of conflict advisory or resolution. A conflict advisory system was developed after the commissioning of the RAMP radar system. The system compared aircraft tracks and altitudes and issued conflict advisories to controllers when it determined that separation standards were likely to be compromised. When the system was tested, deficiencies were identified that required corrective action. Developmental work on a replacement system is reportedly in progress.